1. Field of the Invention
This invention relates to an image forming apparatus in which different image information lights can be applied from a plurality of positions to a photosensitive member for forming an image thereon.
2. Related Background Art
In recent years, attention has been paid to an apparatus for forming an image by effecting image exposure from a plurality of different positions to a photosensitive member, because of its multiple functions.
For example, apparatuses which have a recorded surface on which recording can be effected by writing means and an original carriage for supporting an original thereon and in which both of the copying of the image recorded on the recorded surface and the copying of the original placed on the original carriage are possible are conceived in Japanese Laid-Open Patent Applications Nos. 92626/1976 and 164770/1985. Such image forming apparatuses are very convenient in that the image of the recorded surface is recorded on plain paper by the electrophotographic system and therefore clear-cut images can be obtained and moreover, the copying of either of the original and the recorded image is possible in conformity with the operator's circumstances.
In such apparatuses, however, it is necessary that image lights emitted from a plurality of different positions be accurately aligned with a predetermined position relative to a photosensitive member. This alignment is very difficult and positional deviation may occur, whereby a part of the image may become missing and in some cases, a desired image cannot be obtained.
The following three points may be chiefly mentioned as the causes which promote such positional deviation.
(1) Where the positions at which image information lights are produced differ from each other, a special optical system is more necessary than in the conventional copying apparatus, laser beam printer or the like wherein an image light is applied from a single position.
(2) Where a separate body which emits another image information light is mounted relative to an image forming apparatus body, any error of the mounting appears directly as optical positional deviation. This is more remarkable where the apparatus body and the separate body are removably mountable with respect to each other.
(3) Where the optical lengths until the image information lights arrive at the photosensitive member differ from each other, the difference between the optical lengths leads to a difference in optical accuracy.